Since a negative differential characteristic in a double barrier quantum well was first observed, many studies on adaptation of a resonant tunneling phenomenon capable of transporting an electron at a high speed below 10.sup.-12 second in a high speed device have been accomplished.
FIG. 1 shows a current-voltage characteristic curve of a resonant tunneling diode.
As shown in FIG. 1, a resonant tunneling diode using a resonant tunneling phenomenon has a bistable characteristic having two stable states against a load line due to a negative differential characteristic that a current therethrough decreases regardless of an increase of an applied voltage.
The bistable characteristic has been applied to a memory device, a logic circuit and a high frequency oscillator. Many studies on the bistable characteristic have been accomplished and an oscillation characteristic is disclosed in Appl. Phys. Lett., 58, pp 2291, 1991 by E.R. Brown et al.
The high frequency oscillation device having the resonant tunneling characteristic has an equivalent circuit diagram as shown in FIG. 2.
An input impedance (Z.sub.input) is expressed in the following equation (1) and a cut-off frequency (f.sub.c) has a value of the input impedance at the time that the real part is O and is expressed in the following equation (2). A self-resonant frequency (f.sub.r) has a value of the input impedance at the time that an imaginary part is O and is expressed in the following equation (3): ##EQU1##
The constant, R.sub.s designates a series resistance, R.sub.n negative differential resistance of a tunneling diode, C a static capacitance of the diode, L.sub.s an inductance of a packaged circuit, respectively.
Accordingly, it is noted that a negative differential resistance, a series resistance, a status capacitance and a inductance of a circuit are the primary factors for determining the oscillation characteristic of an oscillator. L.sub.s of the primary factors is determined by the process of manufacturing and it is difficult to change the value of L.sub.s of the previously fabricated oscillator by an external factor.
As shown in FIG. 3, in order to modulate the oscillation characteristic, the prior oscillator having a high frequency characteristic inputs a frequency control signal, synthesizes the frequency of the oscillator and the frequency of the control signal and outputs the synthesized frequency.
That is, because the prior resonant tunneling oscillator utilizes a frequency control method for modulating an oscillation characteristic by an ON/OFF electric signal, restrictions are imposed on a frequency modulation at a ultra-high speed and simplification of a system.